For example, cultivation nets used for culturing fish, such as tuna, yellowtail, or globefish, are generally made of iron or artificial fiber, such as nylon, polypropylene, or polyethylene (for example, Patent Document 1).
Unfortunately, iron cultivation nets (hereinafter referred to as iron nets) and artificial fiber cultivation nets (hereinafter referred to as synthetic nets) easily trap marine organisms, such as (“acorn shells”) and other shellfishes and algae. The marine organisms clog the mesh of the net and thus make it difficult for seawater to pass through the mesh. Consequently, oxygen and nutrients in water cannot be sufficiently supplied to cultivation regions, and thus cultured fish become anorectic. Thus, the productivity and physical strength of the cultured fish are reduced. The cultivation yield is reduced as the resistance to pathogenic bacteria is weakened. Also, parasites, such as gill worms and skin worms, are easily produced. The marine organisms adhering to the net interfere with the behavior of tuna and other migratory fish rubbing against the net. This can adversely affect the growth of cultured fish due to stresses and diseases. Accordingly, it is necessary to frequently remove trapped marine organisms from the net and parasites from the cultured fish. Such work is hard and harsh, and requires extremely high costs.
Furthermore, the iron net is liable to be broken in a relatively short time by corrosion of its wires, because iron being the constituent material of the net has a low corrosion resistance to seawater. Even if only a part of a net is broken, cultured fish can escape from the breakage and this results in considerable losses. The iron net therefore needs to be replaced at regular intervals. The iron net is generally replaced about every two years (or about every year, in some cases). The lifetime of the iron net is thus very short. On the other hand, the synthetic net more easily traps marine organisms, such as shellfishes and algae, than the iron net, and it is accordingly necessary to remove the trapped marine organisms with a frequency of more than or equal to that of the iron net. Although the synthetic net is not corroded by seawater, it inherently has a low shearing strength. Some synthetic nets may result in a shorter lifetime than the iron net depending on circumstances, and may need to be replaced in a shorter time. For replacing a net, cultured fish must be transferred. The replacement of the net not only requires much effort and cost, but also produces adverse effects (for example, stresses) on the cultured fish. The synthetic net also needs to be coated with an antifoulant on a regular basis. The efforts and costs for this work are also high, and the cost for disposing of the waste antifoulant cannot be ignored.
Accordingly, it has been proposed that a cultivation net made of copper alloy wires (hereinafter referred to as the copper net) be used instead of the iron net or synthetic net having the above-described disadvantages (for example, Patent Document 2). In use of the copper net, Cu ions leaching from the wires prevent marine organisms, such as (“acorn shells”), from adhering to the net (this is referred to as “antifouling property”) and sterilize or disinfect the culturing seawater region. Hence, it is not necessary to remove organisms adhering to the net. Accordingly, the efforts and costs for removing organisms can be reduced while adverse effects on cultured fish are eliminated. Furthermore, the sterilization or disinfection of culturing regions can prevent diseases of cultured fish and adverse effects of parasites as much as possible, thus allowing the cultured fish to grow healthily at a high speed.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-337132
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-140677